In air-conditioning apparatuses such as multi-air-conditioning apparatuses for buildings, a refrigerant has been hitherto circulated between an outdoor unit, which is a heat source unit arranged outside a room, and an indoor unit arranged inside the room. The refrigerant has radiated or absorbed heat, and a space to be air-conditioned has been cooled or heated with heated or cooled air. In the case of such a multi-air-conditioning apparatus for buildings, a plurality of indoor units are connected, and indoor units that are not operating and indoor units that are operating often co-exist. Furthermore, since a pipe connecting an indoor unit with an outdoor unit can have a maximum length of as much as 100 m. A system is filled with a larger amount of refrigerant, as the length of a pipe increases.
Such indoor units of multi-air-conditioning apparatuses for buildings are normally placed inside a room used by people (for example, office spaces, living rooms, retail premises, etc.). If a refrigerant leaks from an indoor unit arranged inside a room for some reason, the leakage, depending on the type of the refrigerant, can be a problem of significance, from the viewpoint of safety and harmful effects to humans. In order to address the problem described above, a method in which a two-loop system is employed for an air-conditioning apparatus is known. In the method, air conditioning is performed where a refrigerant is used for a primary loop while water or brine is used for a secondary loop corresponding to an indoor space. In this system, since water, brine, or the like is used for the secondary side, a transfer unit such as a pump is required. If air intrudes into a secondary circuit due to negative pressure or the like of the secondary circuit, air entrainment may occur in operation of a pump, and thus water does not flow. Furthermore, idling run of the pump may cause breakdown of the pump. Under such circumstances, a technique for preventing the pressure of the secondary circuit from becoming negative and preventing air from intruding into the secondary circuit is disclosed.
For example, in Patent Literature 1, by providing an open atmospheric tank including an air-pressure equalizing valve on the pump suction side, the pressure at pump suction is prevented from becoming negative. Furthermore, as in Patent Literature 2, by providing a water-level tank and maintaining the water level of the water-level tank constant, the pressure is prevented from becoming negative.
In Patent Literatures 1 and 2, however, a number of parts increases, which leads to the cost increase, and a tank needs to be installed at a limited position. Thus, the techniques of Patent Literatures 1 and 2 are not suitable as versatile multi-air-conditioning apparatuses for buildings where diverse installations thereof can be assumed.